Fix the searching of target AR tool

Summary:
Ar was being checked twice prior to D3883 where I removed one of the checks
because the converted path was being overridden after the check because of
the second check for Ar. However the one in configure.ac was a target check
so I'm changing the path check to a target check now.

Test Plan: ./configure

Reviewers: angerman, austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, erikd

GHC Trac Issues: #14274

Differential Revision: https://phabricator.haskell.org/D4020

aclocal.m4

@@ -1071,9 +1071,20 @@ AC_SUBST([LdHasFilelist])
 # FP_PROG_AR
 # ----------
 # Sets fp_prog_ar to a path to ar. Exits if no ar can be found
+# The host normalization on Windows breaks autoconf, it no longer
+# thinks that target == host so it never checks the unqualified
+# tools for Windows. See #14274.
 AC_DEFUN([FP_PROG_AR],
 [if test -z "$fp_prog_ar"; then
-  AC_PATH_PROG([fp_prog_ar], [ar])
+  if test "$HostOS" = "mingw32"
+  then
+    AC_PATH_PROG([fp_prog_ar], [ar])
+    if test -n "$fp_prog_ar"; then
+      fp_prog_ar=$(cygpath -m $fp_prog_ar)
+    fi
+  else
+    AC_CHECK_TARGET_TOOL([fp_prog_ar], [ar])
+  fi
 fi
 if test -z "$fp_prog_ar"; then
   AC_MSG_ERROR([cannot find ar in your PATH, no idea how to make a library])
@@ -1151,9 +1162,6 @@ else
   fi
 fi])
 fp_prog_ar_args=$fp_cv_prog_ar_args
-if test "$HostOS" = "mingw32"; then
-    ArCmd=$(cygpath -m $ArCmd)
-fi
 AC_SUBST([ArCmd], ["$fp_prog_ar"])
 AC_SUBST([ArArgs], ["$fp_prog_ar_args"])
 

configure.ac

@@ -609,9 +609,7 @@ dnl ** which libtool to use?
 dnl --------------------------------------------------------------
 # The host normalization on Windows breaks autoconf, it no longer
 # thinks that target == host so it never checks the unqualified
-# tools for Windows. I don't know why we do this whole normalization thing
-# as it just breaks everything.. but for now, just check the unqualified one
-# if on Windows.
+# tools for Windows. See #14274.
 if test "$HostOS" = "mingw32"
 then
     AC_PATH_PROG([LIBTOOL],[libtool])